From reading the threads maybe this is clear to other people but I find the wording of the Spell Perfection feat to be really ambiguous. If it didn't seem that everyone else thinks otherwise it wouldn't be ambiguous at all and would clearly mean the opposite of what people are saying. It says, "Whenever you cast that spell you may apply any one metamagic feat you have to that spell without affecting its level or casting time, as long as the total modified level of the spell does not use a spell slot above 9th level."
Everyone seems to interpret this as saying that you cannot, say, use the feat to put Quicken Spell on a spell of 6th level or greater because that would raise the modified level of the spell to more than 9. But that's not what the feat actually says. It says that the one metamagic feat you apply using doesn't change the level of the spell and it uses a spell slot of the same level as the base spell, unless you put other metamagic feats on it.
As the feat is written, you should be able to use Spell Perfection to put any single metamagic feat you know on to whatever spell you have perfected regardless of level. The caveat would apply only to putting additional metamagic feats on that same spell. I could believe that that's not what the designers intended but that's not what they wrote.